Engine systems may be configured with a boosting device, such as a turbocharger, for providing a boosted aircharge and improving peak power outputs. Therein a turbine is rotated using energy from an exhaust flow, the turbine then driving a compressor which delivers a boosted aircharge to the engine intake. To improve exhaust emissions, engine systems may also be configured with exhaust gas recirculation (EGR) systems wherein at least a portion of the exhaust gas is recirculated to the engine intake. For example, the EGR system may be a low-pressure EGR system (LP-EGR) that recirculates exhaust gas from downstream of an exhaust turbine to upstream of an intake compressor. EGR benefits include an increase in engine dilution, decrease in exhaust emissions, and improvements in fuel economy, especially at higher levels of engine boost.
Introduction of (low pressure) EGR upstream of the compressor requires the compressor inlet pressure to be reduced, so that the EGR can be pulled in from the engine exhaust manifold. The low pressure at the compressor inlet generates a pressure differential across the EGR passage that enables the desired EGR flow to be drawn in. The low compressor inlet pressure may be achieved by throttling the compressor inlet with an additional throttle and/or the air intake system (AIS) throttle. One example of such a system using multiple throttles is shown by Ulrey et al. in U.S. Pat. No. 8,161,746. However, the inventors herein have recognized potential issues with such an approach. As one example, the low pressure at the compressor inlet increases the potential for compressor surge. In addition, durability concerns may be raised if oil from the turbocharger shaft seal is pulled into the turbocharger. Further still, the need for an additional throttle increases component cost as well as complexity in coordinating the control of the additional throttle with the main intake throttle.
In one example, some of the above issues may be addressed by a method for an engine comprising: adjusting an amount of compressor recirculation flow delivered from downstream of a charge air cooler to a compressor inlet via a venturi based on EGR demand. In this way, recirculation flow through a venturi can be advantageously used to generate sufficient vacuum for carbureted EGR flow.
For example, an engine system may be configured with a first compressor recirculation passage that recirculates cooled compressed air from downstream of a charge air cooler to a compressor inlet via a first continuously variable compressor recirculation valve (CRV). A venturi may be positioned in the first compressor recirculation passage downstream of the CRV such that compressed air is recirculated to the compressor inlet upon flowing through the venturi, the flow generating a vacuum at the venturi. The engine system may further include a second continuously variable compressor recirculation passage for recirculating cooled compressed air from downstream of the charge air cooler to the compressor inlet via a second compressor recirculation valve. The second passage may not include a venturi. An EGR passage including an on/off EGR valve for recirculating exhaust residuals from the engine exhaust to the compressor inlet may be coupled only to the first compressor recirculation passage at a location upstream of the venturi (e.g., at the venturi inlet).
During conditions when EGR is requested, the EGR valve may be opened while an opening of the first CRV is adjusted to provide a compressor recirculation flow through the first passage that generates sufficient venturi vacuum for meeting the EGR flow demand. For example, as the EGR flow demand increases, the opening of the first CRV is increased to deliver more compressor recirculation flow through the venturi. At the same time the second CRV may be held closed. In response to an indication of surge, the second CRV is opened to provide surge control while the first CRV is held at the position that maintains EGR flow control.
In this way, EGR can be provided to a compressor inlet in a metered manner. By drawing in the EGR to the compressor inlet using vacuum from a venturi, the need for pre-compressor throttling, including the need for a dedicated throttle is reduced. By enabling EGR to be drawn in without reducing the pressure at the compressor inlet, a margin to surge is also improved. By using compressor recirculation flow through a first passage with a venturi for EGR control while using compressor recirculation flow through a second passage without a venturi for surge control, EGR control and surge control can be concurrently provided. Overall EGR benefits can be provided over a larger engine operating window while boosted engine performance is also improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.